The pump of this invention is in the same field as the pumps shown in the following U.S. Pat. Nos. 2,187,684; 2,636,441; 2,787,225; 3,469,532; 3,502,029; 3,945,772; 4,487,340; 4,762,474; and 6,102,676, the latter of which is directed to a lance pump sold by Lincoln Industrial Corporation of St. Louis, Mo., under the trademark Flow Master®. U.S. patent application Ser. No. 13/331,249 describes another pump in the same general field as the pump of this invention. Although lance pumps such as those identified above have been commercially successful, there is a need for a pump that provides a selectively variable output pressure and reduces a need for complicated reduction gearing.